Japanese Unexamined Patent Application Publications (hereinafter referred to as “Kokai”) 2004-186168 and 2005-327777 disclose curable organopolysiloxane compositions comprising: a branched organopolysiloxane or an oganopolysiloxane having a three-dimensional network structure (hereinafter referred to as “a three-dimensionally networked organopolysiloxane”) that contains in one molecule at least two alkenyl groups; an organohydrogenpolysiloxane having in one molecule at least 2 silicon-bonded hydrogen atoms; and a hydrosilylation catalyst. Since curing of these curable organopolysiloxane compositions forms cured products of high hardness, the compositions find application as protective-coating agents in the manufacture of photocouplers, LEDs, solid-state image sensors, or similar semiconductor elements used in optical semiconductor devices.
However, cured products obtained from the aforementioned known compositions have a high modulus of elasticity and, therefore, have low resistance to thermal shocks and poor adhesive properties. It was proposed, as disclosed in Kokai 2007-063538, 2007-084766, and 2007-182549, to reduce the modulus of elasticity of cured products by introducing linear segments of a diorganopolysiloxane into the branched or three-dimensionally networked organopolysiloxane that contains in one molecule at least two alkenyl groups. In order to introduce linear segments of a diorganopolysiloxane into the branched or three-dimensionally networked organopolysiloxane that contains in one molecule at least two alkenyl groups, it was necessary to produce a diorganopolysiloxane capped at both molecular terminals with diorganochlorosilyl groups by causing a hydrosilylation reaction between a vinyldiorganochlorosilane and a diorganopolysiloxane capped at both molecular terminals with silicon-bonded hydrogen atoms. The obtained diorganopolysiloxane is subjected to co-hydrolysis and condensation with an organotrichlorosilane or a triorganochlorosilane, or a similar chlorosilane, in the presence of an alkali catalyst. However, the organopolysiloxane obtained by the above method contains residual chlorine ions. Furthermore, the alkali catalyst leads to re-arrangement of the linear segments, while the modulus of elasticity of cured products is reduced only insignificantly.
On the other hand, Kokai 2002-088155 discloses a condensation-curable organopolysiloxane composition comprising: an organopolysiloxane that contains silicon-bonded alkoxy groups and is obtained by subjecting a branched or three-dimensionally networked organopolysiloxane having in one molecule at least one alkenyl group and a silicon-bonded alkoxy group and a diorganopolysiloxane capped at both molecular terminals with silicon-bonded hydrogen atoms to a hydrosilylation reaction; a silicon-bonded alkoxysilane; and a condensation catalyst. The last-mentioned reference, however, does not disclose a curable organopolysiloxane composition which is cured by hydrosilylation.
It is an object of the invention to provide a curable organopolysiloxane composition for forming cured products having a favorable modulus of elasticity. It is another object to provide a cured product that has a favorable modulus of elasticity.